20.7.17

PAGING WILLIAM VICKREY?

In Strong Towns, Dan Allison is dubious about high-occupancy vehicle (carpool) lanes.
If some high capacity vehicles are diverted out of general purpose lanes, that provides a more open lane, and that more open lane will be filled with additional traffic. The HOV lane itself, being more open than adjacent lanes, will create additional traffic. Drivers respond to their perception of crowding and delay. If they see more space, they will drive more. It's induced demand, simple as that.

So a HOV lane increases overall traffic. Cost is an issue, as most of our transportation dollars at the state and regional level go to these projects, instead of projects that would actually reduce private vehicle use and vehicle miles traveled. Environmental and social impacts increase. And the lanes fill up, creating a demand for yet more lanes in a never-ending cycle.
His complaint is with building new capacity that's rationed by congestion, rather than new capacity that's rationed by price.  Where there are new lanes, traffic will increase to fill the available capacity.  The flaw is in rationing the capacity by congestion, and the carpool lane works for people who gain enough by going faster to offset the costs of setting up the carpool (or ride share.)  It's in the incentives, people.  "There has to be an efficient way of pricing a congestible facility in such a way that both the premium-price for no waiting and the low price wait your turn riders have no incentive to change their types."

Note, though, it's about the pricing.
As a modeling exercise, the marginal commuter is indifferent between the marginal utility adjusted by the higher price of the premium service and the marginal utility adjusted by the lower price of the congested service (there are some additional subtleties involved in avoiding division by zero.)
If the carpool lane becomes a high-occupancy toll lane, or simply a toll lane, it's straightforward to price the use in such a way as to cover the incremental cost of the additional capacity.  Yes, there are more vehicle miles being travelled, and yes, the analyst must still consider the network effects. A new toll lane induces more traffic onto the existing expressway until the existing expressway is as slow as it used to be, but those travellers who are now congesting the expressway were congesting some other road instead.

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